The present invention relates to a vented nuclear fuel element, particularly to one which can discharge gaseous fission products released from nuclear fuel after a holdup period of a sufficient long time, without introducing the coolant surrounding the fuel element into the nuclear fuel.
Some conditions that are necessary for a vented nuclear fuel element of a fast breeder reactor in which liquid metal, particularly sodium, is the coolant, ar as follows: (1) It must be able to prevent the admission of the liquid metal coolant into the fuel element portion containing the nuclear fuel during its lifetime, because the contacting of the nuclear fuel, usually (PU-U)O.sub.2, and the liquid metal coolant, usually Na, is undesirable; and (2) It should be able to prevent pollution of the cooling system and cover gas system by radioactivity.
In order to minimize the pollution, it is necessary for the nuclear fuel element to holdup the gaseous fission products therein for a period of time sufficient for the products to become stable or comprise only long half-life fission products, which products may be considered harmless with respect to radioactivity. Also, the components of the venting device must be able to withstand the difficult conditions of temperature, ratiation, heat cycling, and the like are encountered in a nuclear fuel element.
In the prior art, it is known to provide a diving bell type venting device having a vent tube extending upwardly to the interior of the upper portion of a cladding tube and a plurality of holes provided for the cladding tube near the lower end of the vent tube. This venting device has a high reliability, since it is simple in its construction and is not influenced by changes in its material property due to eradiation. However, the venting device has a disadvantage such that the device must be of a considerable length and quite large in its construction, in order to holdup gaseous fission products for a long time or in order to perfectly prevent admission of the surrounding coolant into the interior of the fuel element.
Another diving bell type of venting device, according to the prior art, has another vent tube extending downwardly from the top of the cladding tube, in addition to the above mentioned vent tube that extends upward. The top portion of the vent tube that extends downward is pinched and exposed in the surrounding coolant. The pinched portion has a function of raising the pressure in the interior of the fuel element, which function is caused by the spring back of the vent tube. This device has a simple construction, however, it has drawbacks, for example, radiation to the vent tube may result in damaging the material so as to impair the spring back function, and further solid materials included in the gaseous fission products may become inserted or wedged into the pinched portion, which will result in failure of the pinched portion function.
Further, the prior art includes a venting device for a nuclear fuel element, which comprises a porous plug closing the top end of the fuel element. This venting device is excellent with respect to economy, because of its minimum construction. However, the device has problems in that it is not able to prevent admission of the surrounding coolant into the fuel element by a gradual change of the surface tension of the porous plug, in the case where the device is inserted in a core for a nuclear reactor for a long time.